The field of the invention relates generally to fluorescent lamps and more particularly, to fluorescent lamps having a multi-layer phosphor coating that provides a high color rendering index with acceptable brightness at economical cost.
Fluorescent lamps may employ various combinations of three or four narrow band emitting phosphor materials for conversion of UV light to visible light, each phosphor material producing a red, a blue or a green color emission. The combination of phosphor materials is useful in low pressure type mercury vapor discharge lamps. Typically, a blue color emission phosphor exhibits an emission band in the wavelength range from about 430 nanometers wavelength up to about 500 nanometers wavelength, a red color emission phosphor exhibits an emission band in a wavelength range from about 590 nanometers wavelength up to about 670 nanometers wavelength, and a green color emission phosphor producing emission extending from about 500 nanometers wavelength up to about 600 nanometers wavelength. This phosphor combination produces efficient white illumination. A blend of four narrow band emitting phosphor materials may include first and second green emitting phosphors, having different visible emission spectrum, a third blue emitting phosphor, and a fourth red emitting phosphor. Some known phosphor combinations have employed a dual layer phosphor coating. Specifically, a thin layer of the tri-phosphor combination may be deposited on the surface of a basecoat of conventional calcium haloapatite phosphor material to produce the desired high light output with fair color rendition at a considerable cost savings for the overall phosphor combination. But in these known dual phosphor coatings, the color rendering index (CRI) of the basecoat is typically about 70, and the triphosphor top layer typically has a CRI of about 83; that is, the CRI of the basecoat is always much lower than that of the topcoat. The combination of the layers results in a CRI of about 78. There continues to be a need for further energy efficient lamps that reduces the cost of relatively expensive phosphor materials.